libafs: Fix statistics for PrefetchFromTape pioctl

The PrefetchFromTape pioctl was overloading SetAcl's statistics. Give
it a slot of its own.

1.6-only: Also give a slot for the master-only PSetTokens2, so xstat
data will be more consistent across versions.

Reviewed-on: http://gerrit.openafs.org/4989
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 4e9a49c58aa1a6c910b01fe995739d0852c75dd4)

Change-Id: I718585a6ff6fcb39469bd6a007e6ebd2b59ef4fa
Reviewed-on: http://gerrit.openafs.org/9468
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>